1. Field of the Invention
The present invention relates generally to a satellite positioning receiver, and more particularly to a Global Positioning System (GPS) receiver in an assisted GPS handset operating in conjunction with one or more cellular base stations (BSs).
2. Description of the Background Art
The Global Positioning System (GPS) is a satellite-based positioning system developed by the U.S. Department of Defense to give accurate positional information to a GPS receiver anywhere in the world. A properly equipped GPS receiver may therefore be used in any setting in which a positional fix is desired, and typically yields positional coordinates in three dimensions.
GPS is enabled by a satellite orbital constellation made up of twenty-four or more satellites orbiting the earth in twelve-hour orbits. The satellites are arranged in six orbital planes, each containing four satellites. The orbital planes are spaced sixty degrees apart and are inclined approximately fifty-five degrees from the equatorial plane. This constellation ensures that from four to twelve satellites will be visible at any time at any point on earth with a clear view of the sky.
A three-dimensional position fix may be determined if signals are being received from four or more GPS satellites. The received satellite signals each contain an identifier unique to that particular satellite. These identifier codes are commonly called Gold codes or pseudorandom noise (PRN) codes and allow a GPS receiver to discriminate between signals from different satellites. Also contained within the signals are satellite ephemeris data containing information such as an orbital configuration and a satellite time. (All GPS satellite signals contain a common, synchronized GPS time.) This satellite time signal allows a GPS receiver to detect a time of receipt and therefore measure a transit time of the signal. In turn, the transit time enables a GPS receiver to determine a distance (termed a pseudorange) to the satellite. Four GPS pseudoranges allow a location on the Earth""s surface to be determined.
There are many uses for portable, hand-held GPS receivers including use in an assisted GPS handset. Because of processing time constraints and incomplete satellite coverage, handset-based GPS receivers may operate in conjunction with one or more BSs. As described in related art, the base station (BS) can be employed to relieve GPS receivers from certain processing duties, improve the accuracy of the resulting location solution, and improve fix coverage in areas that are marginal for GPS signal tracking (e.g., in an urban canyon or indoors). The serving BS (i.e., the BS that is responsible for maintaining communication with the handset) can provide the mobile handset with satellite Doppler information (U.S. Pat. No. 4,445,118 to Taylor, et al.) and/or satellite ephemeris data (U.S. Pat. No. 5,365,450 to Schuchman, et al.) to reduce the time required to acquire the GPS signals and derive a location fix within the handset. In addition, differential corrections can be broadcast to GPS receivers in handsets. These corrections, if received with minimum latency, can remove most of the error associated with each pseudorange measurement. Finally, additional range measurements can be derived from the arrival times associated with the transmissions of pilot signals from each BS, which can be used to augment the GPS-only solution and provide position fixes in situations where GPS cannot. One type of information that is commonly relayed in the related art is locations of nearby BSs. BS locations may be useful to the mobile GPS receivers to give a starting position for a positional fix and may reduce the signal search space and therefore reduce the acquisition time for the GPS signals. A second type of information that is commonly relayed is a time bias offset associated with each nearby BS. This offset is a bias between the time maintained by the BS and GPS time.
FIG. 1 shows a related art mobile GPS receiver in a cellular handset operating in conjunction with a single BS. Each related art cellular handset receives a transmission from the BS containing the BS position embedded therein along with a time bias offset b. The BS position may be expressed in Cartesian coordinates as xBS, yBS. A drawback of the related art approach is that related art handset-based GPS receivers cannot make use of TOA measurements from the network infrastructure and calculate a positional fix based solely on TOA measurements until the required data (xBS, yBS, b) is received from the BS.
What is needed, therefore, is a method for determining BS locations and time bias offsets in a handset-based GPS receiver without necessarily having to receive the BS locations and time bias offsets from the infrastructure.